The stabilization of metals in hazardous wastes has been done at remedial operations and at fixed treatment sites for more than 20 years in the U.S. It is an accepted method of treatment, specified in 22% of the 1990 Superfund Records of Decision (ROD).sup.1 and as Best Demonstrated Available Technology (BDAT) for 44 EPA listed wastes under the Land Disposal Restrictions (LDR).sup.1. Commercial waste treatment companies have been running stabilization operations for metals at their RCRA Treatment, Storage and Disposal Facilities (TSDF) since 1985, treating literally thousands of different waste streams. The immobilization (or stabilization, chemical fixation, etc.) of organic constituents in wastes, although more recent, has been shown in numerous studies.sup.23 to be capable of considerably reducing the mobility of these hazardous contaminants as measured by the Toxicity Characteristic Leaching Procedure (TCLP).sup.4 The TCLP has been the standard regulatory leaching test used in the United States for many years. Immobilization of low levels of hazardous organic compounds in soils, sludges, debris and other wastes has received increasing regulatory attention of late. In addition to several older guidelines under "Superfund" (CERCLA), two recent final regulations under the Resource Conservation and Recovery Act (RCRA) mandate treatment of all hazardous organics contained in "contaminated debris" and of 26 compounds contained in other wastes.
Under CERCLA, a variance procedure in remedial actions allows immobilization of organics as an alternative to removal or destruction.sup.5, and a draft guidance document from EPA Risk Reduction Engineering Laboratory (RREL).sup.6 that recommends total constituent analysis be used as a means of judging the success of immobilization. Under RCRA, the 1992 "debris rule".sub.7 and the new rule dealing with EPA Waste Codes D018-D043 ("Characteristic" or "D-Code" wastes).sup.8 require the treatment of hazardous constituents in debris and other wastes. Most importantly, now two different test methods are used to judge the effectiveness of the treatment in meeting the regulatory requirements: the Toxic Characteristic Leaching Procedure (TCLP) and Total Constituent Analysis (TCA). TCA has replaced TCLP in the case of organics for most purposes under RCRA, and is recommended.sup.6 as the primary test in CERCLA and other remedial actions. It is also the basis for EPA's Universal Treatment Standards (UTS).sup.9 TCLP is used in the case of debris.sup.7 and as an additional test in remedial work.
Due to the test procedures, meeting the present and forthcoming TCA standards for organics is much more difficult than passing the TCLP test. Previously, the use of reagents such as activated carbon in stabilization systems to immobilize organic constituents was based on the TCLP test method. However, with the TCA test method, such reagents are often not very effective. In the TCLP test, a dilute aqueous solution of acetic acid or buffered acetic acid at pH 2.88 or 4.93, respectively, is used depending on the alkalinity of the waste sample to be tested. The extraction solution to sample ratio is 20:1 by weight, and the waste particle size is less than 9.5 mm. The waste and extraction solution are mixed together and agitated for a period of 18 hours, after which the mixture is filtered and the filtrate analyzed for constituents of concern by standard EPA methods.sup.10. The purpose of the test is to determine to what extent a contaminant will leach from a waste under what EPA called an "improperly managed" disposal scenario, i.e., a reasonable worst case condition in a co-disposal landfill. The amount leached will depend on the properties of the constituent, waste form properties and test factors: solubility, diffusion through the matrix, particle size, etc. Stabilization processes have historically been designed to resist the leaching effect of tests such as this.
The TCA test, strictly speaking, is not a performance test but an assay procedure. In other words, it was intended to be used to measure the total amount of a constituent in the waste, not its mobility. In the case of metals in a solid sample, the sample is completely dissolved, or "digested," in various strong acids so that all constituents go into solution and can then be analyzed. The test result accurately reflects the total amount of metal present in the original sample. In the case of organics, however, the metal digestion method cannot be used because many compounds would be destroyed or altered by the procedure itself. Nevertheless, it is necessary to get the organic out of the solid and into solution so that it can be analyzed for. This is done by means of extractions with one or more powerful solvents that are presumed to dissolve out essentially all of the organic from the solid phase matrix. However, it is a fact that any multi-phase extraction process will partition the constituent being extracted according to the relative affinity of the constituent for the respective phases. This is expressed as a partition coefficient (amount in the liquid phase divided by the amount in the solid phase, after extraction), a term quite familiar in the chemical process industries. For analytical purposes, the partition coefficient must be very high. If the organic constituent was found at significantly lower concentrations, or not detected at all in the TCA test, it was assumed that it had either been volatilized, destroyed, or converted into another species, most likely volatilized. This is probably the basis for EPA's opinion that stabilization is not appropriate for many organics, since they were assumed to have been driven off during the mixing action in stabilization and/or by the heat generated in the chemical reactions.
Before EPA arrived at its new stance in using TCA instead of TCLP for measuring hazardous organic constituents, a testing program was conducted.sup.11 on the stabilization of organics using the TCLP test. In testing various reagents in a cement-based stabilization system for their ability to immobilize organics as measured by the TCLP test.sup.3, we also measured TCA before and after treatment to establish a materials balance--to determine whether the organic constituent was retained in the treated matrix, and not volatilized. Unexpectedly, it has been found that the rubber particulate not only immobilized the organics, but that in the majority of cases the semi-volatile organics (SVOCs) were no longer detectable.sup.1 by the TCA test in the treated, solid, waste mixture itself. VOCs, on the other hand were not reduced in concentration in most cases by rubber particulate alone, with only several of the compounds tested being substantially reduced in concentration. Furthermore, it was found that rice hull ash (RHA), which was not effective in TCA reduction with SVOCs, was very effective with VOCs; the majority of the constituents tested showed substantial TCA reductions. Together, rubber particulate and rice hull ash were capable of substantial TCA reduction for about 80% of the compounds tested. In comparison, and as was originally expected, organics in the samples treated by cement alone, cement plus activated carbon or cement plus organically-modified clay were not substantially reduced--less than 25% effectiveness in either case. .noteq..sup.1 Detection levels vary for different compounds in different matrices, but are generally in the range of 0.1 to 10 mg/kg (ppm)
The exact mechanism for this phenomenon is not known at this time. We do know, however, by the way in which the experiments were conducted, that the organics were not vaporized or otherwise lost to the environment. The organics may have been chemically modified, or bonded in some way to the rubber so that they could not be separated by the analytical procedure. Scientists have hypothesized that such immobilization may involve some sort of chemical bonding or even conversation of the constituent into another compound. The heat of adsorption, .DELTA.H.sub.ad, is a measure of the binding energy between the substrate and an adsorbate.sup.12. When .DELTA.H.sub.ad is .ltoreq..about.15 kcal/mol, a weakly adsorbing species, it is known as physisorption; when .DELTA.H.sub.ad is .ltoreq..about.15 kcal/mol, a strongly adsorbing species, the bond is more chemical-like and is termed chemisorption. However, this is a continuum, not a sharp dividing line. Certainly, more than simple, reversible surface adsorption is involved. Vendors of organo-clays have claimed for years, with some support from others, that chemical reactions and/or chemisorption are taking place on the active, interior surfaces of reagent particles. This may explain the ability of micro-porous materials like rubber particulate and RHA to achieve immobilization as determined by the TCA test.
In any case, by EPA definition the organics are not longer present or are not present in sufficient quantity to pose a hazard to human health and the environment. We are not aware of any known properties of rubber or rice hull ash that account for this discovery, nor is it disclosed or suggested in any known environmental or technical literature. In addition to their vastly superior ability to destroy or bond organics, ground rubber and RHA cost only one-third to one-half as much as activated carbon, and like carbon, are not biodegraded so that they are stable when landfilled. This is not necessarily true of organically-modified clays or many other organic sorbents. The stability of rubber in landfills is well proven and documented. RHA is mostly amorphous silica, and is known to be non-biodegradable.
The test method prescribed by the new regulations and guidances has significantly, suddenly and unexpectedly changed the whole stabilization picture for organics. Assumptions made on the basis of many years of using activated carbon and other carbonaceous sorbents in the treatment of air emissions and water are not valid in this situation. When the TCLP test was used to test for immobilization, carbon was the most generally effective commercial reagent. With the TCA test, carbon is ineffective for most compounds, even for VOCs where it most stood out in the TCLP testing. Reductions in TCA using the process of the present invention ranged up to 99.9%, with some reduction in all cases. Reductions in TCLP leachability ranged from a minimum of 90% to better than 99%. Now, immobilization of low-level organics in soils, sludges and debris is feasible.
The use of shredded or particulate rubber and the like for physically absorbing organic solvent and fuels is well known. U.S. Pat No. 4,182,677 (Bocard et al, Jan. 8, 1980) relates to a process for absorbing hydrocarbons or organic solvents, particularly in solution or suspension in water at low concentration, by means of an absorption mass consisting of rubber particles of from 0.1 to 3. mm, subjected to a treatment with an organic or inorganic acid or an aqueous solution or emulsion thereof. U.S. Pat No. 3,567,660 (Winkler, Mar. 1971) relates to a process for converting oil spills into a useful material, using rubber and asphaltic material. U.S. Pat. No. 2,333,142 (Behrman, Nov. 2, 1943) relates to a water softening process using sulfonated rubber particulate for an ion exchange material. U.S. Pat. No. 4,728,343 (Snyder, Mar. 1, 1988) relates to a method of substantially precluding the accumulation of combustible organic vapors in a storage container by placing comminuted vulcanized rubber as an absorbing medium in contact with the vapors.
The use of rice hull ash as an absorbent for hazardous wastes is also well known. U.S Pat. No. 4,460,292 (Durham, Jul. 1984) describes the burning of biogenetic materials such as rice hulls to produce amorphous, biogeneric silica, rice hull ash, and its use as a physical absorbent for hazardous wastes. U.S. Pat. No. 5,078,795 (Conner et al, Jan. 7, 1992) discloses the solidification and chemical fixation of wastes using rice hull ash and an alkali and a polyvalent metal ion to provide a cementitious product of the waste.
For the purposes of the present application the terms volatile organic chemicals or materials (VOC) and semi-volatile chemicals or materials (SVOC) are distinguished by the EPA's classification which is set forth in reference 8 below. Such regulation includes a complete listing of hazardous and regulated organics subject to the regulation and for which the method of the present invention is applicable. Such regulations and listing are hereby included in this specification by reference. Additions to such listings made hereafter are also included.